Anodization process and layers produced therefrom

ABSTRACT

A multi-step anodization process is disclosed for forming a barrier layer on a photoreceptor substrate. The process produces layers in relatively short periods of time. Also disclosed is a preheating operation that produces a relatively hard anodized layer. The preheating operation can be used in conjunction with either of the multi-step anodization process, or with conventional processes.

BACKGROUND

The present disclosure, in various exemplary embodiments, relates to afast anodization process that produces a hard anodized barrier layeradapted for photoreceptor substrates.

Electrostatographic imaging systems, which are well known, involve theformation and development of electrostatic latent images on an imagingsurface of an electrostatographic or photoreceptor. Xerographicphotoreceptors can be prepared in either a single-layer or a multilayerconfiguration. Depending on the application, the photoreceptors can beprepared in several forms, such as flexible belts, cylindrical drums,plates, etc. Belts are usually prepared on polymer substrates,poly(ethylene terephthalate) being the most common. For drums, thesubstrate is typically a metal cylinder. Usually, hollow aluminumcylinders are widely used in low- and mid-volume applications. The drumconfiguration, however, has certain process limitations for high-volumeand color applications.

Photoreceptors are prepared by the sequential application of variouslayers (i.e., charge generating layer, charge transport layer, etc.)onto the outer surface of a polymer or drum substrate. Many coatingtechniques (i.e., spraying, spinning, extrusion, dipping, blade coating,roll coating, etc.) may be utilized to produce these layer(s). Vapordeposition may also be used for metallization and application of somepigments.

At present, long life photoreceptors free of defects and carbon fiberproblems cannot be made. An electrolytic barrier layer on the substratehowever, can extend the life of a photoreceptor. However, such a barrierlayer typically requires up to 15 minutes to apply thereby making itunattractive for commercial manufacture. Another disadvantage to anelectrolytic barrier layer, relates to the barrier's insufficienthardness, and thus, poor carbon fiber roboustness.

Extended photoreceptor life, such as for example an increase in life oftwo to ten times, and robustness is presently achieved as a result ofusing, in part, an electrolytic barrier layer, and particularly oneformed by low temperature controlled voltage and organic acidanodization. While the process appears to be substantially complete inless than one minute, mixed results regarding the extent of defects,thus, extended life are obtained if the process is terminated at theone, two, five, and ten-minute points. It has been determined thatconsistent results are obtained by continuing the application of voltagefor times exceeding ten minutes. It is believed that additionalimpurities are removed from the aluminum photoreceptor substrate surfaceduring the extended process time. When such impurities are left in placeand not removed, these impurities act as sites for the initiation ofdefects. However, times in excess of one minute are not attractive tomost commercial manufacturing operations.

Accordingly, it would be desirable to provide a process that reliablyfacilitates extended life but that could be performed in one minute orless.

BRIEF DESCRIPTION

The present disclosure concerns, in various exemplary embodiments, aprocess for forming a barrier layer on a photoreceptor substrate byorganic acid anodization. The process comprises providing aphotoreceptor substrate and providing an organic acid electrolyte. Theprocess further comprises contacting the photoreceptor substrate withthe electrolyte. The process also comprises applying a multi-stepvoltage profile to the photoreceptor substrate in contact with theelectrolyte. The profile includes a first step in which a first voltageis applied for a first time period and a second step in which a secondvoltage, less than the first voltage, is applied for a second time.

In another exemplary embodiment, a process for forming a barrier layeron a photoreceptor substrate by organic acid anodization is provided.The process comprises providing a photoreceptor substrate and providingan organic acid electrolyte. The process comprises heating thephotoreceptor substrate to a temperature of from about 450° C. to about650° C. The process also comprises contacting the photoreceptorsubstrate with the electrolyte. And, the process comprises applying avoltage to the photoreceptor substrate in contact with the electrolytefor a period of time so as to form an anodized layer thereon.

Still further advantages and benefits of the present exemplaryembodiments will become apparent to those of ordinary skill in the artupon reading and understanding the following detailed description of thepreferred embodiments.

DETAILED DESCRIPTION

The exemplary embodiment provides a fast and low voltage anodizationprocess to form an electrolytic barrier layer for substrates such asthose for organic photo conductor photoreceptors. The process uses areduced voltage profile, such as from about 24 volts to about 12 volts,to reduce the process time from 15 minutes to as short as 42 seconds.The resulting anodized barrier layer extends the functional life oforganic photoconducting photoreceptors and prevents defects caused bycarbon fiber penetration of the coated layers.

The exemplary embodiment provides a process for forming an electrolyticbarrier layer using a multi-step voltage profile. A two-step profile canutilize a first step in which a first voltage is applied for a firsttime period and a second voltage, less than the first voltage is appliedfor a second time period. The second voltage is from about 50% to about20% of the first voltage, and particularly, from about 40% to about 30%of the first voltage. Representative voltages for the first step can befrom about 20 to about 24 volts, and for the second step can be fromabout 12 to about 17 volts. The total time for the two-step profile,i.e. the sum of the first and second time periods, is less than about 5minutes, and particularly less than about 1 minute.

A three-step profile can utilize a first step in which a first voltageis applied for a first time period, a second voltage, less than thefirst voltage, is applied for a second time period, and a third voltage,less than the second voltage, is applied for a third time period. Thesecond voltage is from about 40% to about 30% of the first voltage. Thethird voltage is about 30% to about 20% of the second voltage.Representative voltages for the first step can be from about 22 to about26 volts, for the second step from about 14 to about 18 volts, and forthe third step from about 10 to about 12 volts.

By starting the process at higher voltages and stepping down the voltageduring the process, it is possible to quickly obtain the same barrierlayer as one formed from a lower voltage applied for a longer time. Forexample, starting at 24 volts for 10 seconds then stepping down to 16volts for another 12 seconds followed by another 20 seconds at 12 voltsproduces the same barrier layer that is obtained using 12 volts for 15minutes but only takes 42 seconds. Thus, the process is now compatiblewith a manufacturing process step window of one minute.

Barrier layers having thicknesses of twenty nanometers formed at 12volts and thirty nanometers (formed at 17 volts) were produced in a 1%w/w citric acid electrolyte at 14° C. in one minute or less by startingthe barrier layer process at 20 and 24 volts respectively for 15 secondsfollowed by 25 seconds at 12 and 17 volts respectively. These barrierlayers were tested and found to be similar to barrier layers producedwhen 12 and 17 volts were used for 15 minutes.

In addition to these two-step voltage processes, three-step voltageprocesses also produce layers with excellent characteristics. Indeed,while undemonstrated, it is believed that a continued or gradual rampingreduction of the voltage would produce comparable results.

Note that just maintaining a higher voltage for a shorter time producesa barrier layer that is too thick for use with many photoreceptors, thusproducing a residual voltage in excess of 100 volts.

Although not wishing to be limited to any particular theory, it isbelieved that the more aggressive starting voltage is deep cleaning thesubstrate surface during the first few seconds, without sufficient timeto form a barrier layer that is too thick, followed by the production ofa useable barrier layer at the reduced voltage.

The exemplary embodiment also provides an anodization process,specifically using a heat pretreatment that produces a hard anodizedbarrier layer on a substrate such as used with organic photoconductorphotoreceptors. The resulting hard layer extends photoreceptor life andreduces the carbon fiber penetration problem. The exemplary embodimentcan utilize 550° C. heat pretreatment. The heat treatment is believed tocreate crystalline oxide “seeds” that propagate during the electrolytic(anodizing) step, causing the formation of a harder anodized barrierlayer.

By subjecting the naturally occurring aluminum oxide on a photoreceptorsubstrate to elevated temperatures before the member is anodized, thebarrier layer becomes a crystalline versus the normal amorphous form.This crystalline variant is much harder than the amorphous form.

More specifically, the exemplary embodiment process utilizing a heatingpretreatment can be utilized in conjunction with conventional processesfor forming an electrolytic barrier layer, and more particularly, withthe accelerated processes described herein. The exemplary embodimentheat treatment features subjecting a member to receive a barrier layer,such as a photoreceptor, to temperatures from about 450° C. to about650° C., and particularly about 500° C. The time period for such heatingcan vary, however, times of from about 10 seconds to about 60 seconds,and particularly about 30 seconds can be utilized.

The cleaned photoreceptor substrate is first subjected to a 30 secondheat treatment at 550° C. then anodized at 10 volts for 15 minutes in a1% citric acid electrolyte at 14° C. Alternatively, the exemplaryembodiment includes the use of 16 volts for 15 seconds followed by 10volts for 25 seconds. These processes will produce a crystalline barrierlayer that has an equivalent capacitance to the barrier layer producedat 12 volts in the same system after 15 minutes.

While this heat treatment described herein does not cause the naturaloxide to grow very much, it is believed that the heat treatment causesthe formation of “seeds” of crystalline oxide that propagate during thesubsequent electrolytic (anodizing) step.

A general description of the electrolytic process now follows. In theelectrolytic cell, the working electrode is the photoreceptor substrate(Anode). The counterelectrode can be concentric, generally surroundingthe exterior of the substrate. To simultaneously clean the interior ofthe substrate in the case of a cylindrical substrate, a concentriccounter electrode may be disposed in the interior of the substrate. Thecounterelectrode or electrodes may be a noble metal such as gold,silver, platinum, palladium; an inert material such as graphite; or astrongly passive material such as titanium, lead, tantalum, or alloysthereof. The cell voltage is modulated with a power source capable ofdelivering direct voltage.

The electrolyte used may be any of several acids. These include citricacid monohydrate, oxalic acid dihydrate, and d-tartaric acid.Preferably, citric acid is used at 0.5 and 1.0 w/v % (pH 2). Oxalic acidis used at 0.5, 0.62, and 1.0 w/v % (pH 1). D-tartaric acid is used at0.5 and 1.0 w/v % (pH 2.5). The temperatures used are from 13 to 18° C.These baths are not generally sensitive to concentration variability andonly slightly sensitive to temperature changes in these ranges. Notethat these characteristics make these baths attractive as well formanufacturing (low concentrations and robust to operating parameterchanges). In addition to these organic acids, inorganic acids commonlyused for anodizing like sulfuric acid, chromic acid, et cetera can alsobe used. Generally, citric acid is preferred as it is the mostenvironmentally friendly. The voltages used are generally in the rangefrom 8 to 24 volts.

In embodiments, there is formed a metal oxide layer on the substratesurface wherein the metal oxide layer added by the exemplary embodimentshas a thickness ranging from about 50 to about 200 angstroms, and moreparticularly from about 70 to about 150 angstroms. The metal oxide maybe for example aluminum oxide. The actual thickness is difficult tomeasure. Hence, in an alternate, the so-called, V_(low) of the finishedphotoreceptor is measured and adjusted in the process to keep theV_(low) to less than 100 Volts. The final current passing at the end ofthe anodizing process is used as a surrogate to know that a good barrierlayer has been obtained. Note that this final current is dependent onseveral factors in addition to the thickness of the barrier layer. Theseother factors are, therefore, preferably kept constant and include: thenumber and size of the parts being anodized, the rack (holds the parts)configuration, and to a lesser extent the temperature of theelectrolyte. Note also, that sufficient electrolyte movement (mixing) toinsure uniform temperature (+/−1° C.) is preferred.

The substrate preferably is a hollow cylinder and defines a topnon-imaging portion, a middle imaging portion, and a bottom non-imagingportion. The precise dimensions of these three substrate portions varyin embodiments. As illustrative dimensions, the top non-imaging portionranges in length from about 10 to about 50 mm, and particularly fromabout 20 to about 40 mm. The middle imaging portion may range in lengthfrom about 200 to more than 1000 mm, and particularly from about 250 toabout 300 mm. The bottom non-imaging portion may range in length fromabout 10 to about 1 mm, and particularly from about 5 to about 10 mm.The substrate may be bare of layered material or may be coated with alayered material prior to immersion of the substrate into the coatingsolution.

The substrate can be formulated entirely of an electrically conductivematerial, or it can be an insulating material having an electricallyconductive surface. The substrate can be opaque or substantiallytransparent and can comprise numerous suitable materials having thedesired mechanical properties. The entire substrate can comprise thesame material as that in the electrically conductive surface or theelectrically conductive can merely be a coating on the substrate. Anysuitable electrically conductive material can be employed. Typicalelectrically conductive materials include metals like copper, brass,nickel, zinc, chromium, stainless steel; and conductive plastics andrubbers, aluminum, semitransparent aluminum, steel, cadmium, titanium,silver, gold, paper rendered conductive by the inclusion of a suitablematerial therein or through conditioning in a humid atmosphere to ensurethe presence of sufficient water content to render the materialconductive, indium, tin, metal oxides, including tin oxide and indiumtin oxide and the like. The coated or uncoated substrate can be flexibleor rigid, and can have any number of configurations such as acylindrical drum, an endless flexible belt, and the like. This (groundstrip) should preferably be anodizable (Ti, Al, et cetera) or addedafter the anodize process.

The layers of the substrate member can vary in thickness oversubstantially wide ranges depending on the desired use of thephotoconductive member. Generally, the conductive layer ranges inthickness of from about 50 Angstroms to 10 centimeters, although thethickness can be outside of this range. If desired, a conductivesubstrate can be coated onto an insulating material. In addition, thesubstrate can comprise a metallized plastic, such as titanized oraluminized MYLAR® (available from DuPont). The coated or uncoatedsubstrate can be flexible or rigid, and can have any number ofconfigurations. The substrates preferably have a hollow, cylindricalconfiguration.

The layer of a photosensitive member including such layers as a subbinglayer, a charge barrier layer, an adhesive layer, a charge transportlayer, and a charge generating layer, such materials and amounts thereofbeing illustrated for instance in U.S. Pat. No. 4,265,990, U.S. Pat. No.4,390,611, U.S. Pat. No. 4,551,404, U.S. Pat. No. 4,588,667, U.S. Pat.No. 4,596,754 and U.S. Pat. No. 4,797,337, the disclosures of which aretotally incorporated by reference. These layers may be apparent by knowncoating processes.

In certain embodiments, the coating solution may be formed by dispersinga charge generating material (CGL) selected from azo pigments such asSudan Red, Dian Blue, Janus Green B, and the like; quinine pigments suchas Algol Yellow, Pyrene Quinone, Indanthrene Brilliant Violet RRP, andthe like; quinocyanine pigments; perylene pigments; indigo pigments suchas indigo, thioindigo, and the like; bisbenzoimidazole pigments such asIndofast Orange toner, and the like; phthalocyanine pigments such ascopper phthalocyanine, aluminochlorophthalocyanine, and the like;quinacridone pigments; or azulene compounds in a binder resin such aspolyester, polystyrene, polyvinyl butyral, polyvinyl pyrrolidone, methylcellulose, polyacrylates, cellulose esters, and the like.

The average particle size of the pigment particles is between about 0.05micrometer and about 0.10 micrometer. Generally, charge generating layerdispersions for immersion coating mixture contain pigment and filmforming polymer in the weight ratio of from 20 percent pigment/80percent polymer to 80 percent pigment/20 percent polymer. The pigmentand polymer combination are dispersed in solvent to obtain a solidscontent of between 3 and 6 weight percent based on total weight of themixture. However, percentages outside of these ranges may be employed solong as the objectives of the process of this disclosure are satisfied.A representative charge generating layer coating dispersion comprises,for example, about 2 percent by weight hydroxy gallium phthalocyanine;about 1 percent by weight of terpolymer of vinyl acetate, vinylchloride, and maleic acid (or a terpolymer of vinylacetate, vinylalcoholand hydroxyethylacrylate); and about 97 percent by weight cyclohexanone.

In other embodiments, the coating solution may be formed by dissolving acharge transport material (CTL) selected from compounds having in themain chain or the side chain a polycyclic aromatic ring such asanthracene, pyrene, phenanthrene, coronene, and the like, or anitrogen-containing hetero ring such as indole, carbazole, oxazole,isoxazole, thiazole, imidazole, pyrazole, oxadiazole, pyrazoline,thiadiazole, triazole, and the like, and hydrazone compounds in a resinhaving a film-forming property. Such resins may include polycarbonate,polymethacrylates, polyacrylate, polystyrene, polyester, polysulfones,styrene-acrulonitrile copolymer, styrene-methyl methacrylate copolymer,and the like.

An illustrative charge transport layer coating solution contains, forexample, about 10 percent by weightN,N′-diphenyl-N,N′-bis(3-methylphenyl)-[1,1′-biphenyl]-4,4′diamine;about 14 percent by weight poly(4,4′-diphenyl-1,1′-cyclohexanecarbonate) (400 molecular weight); about 57 percent by weighttetrahydrofuran; and about 19 percent by weight monochlorobenzene.

Furthermore, the charge generating layer, charge transport layer, and/orother layers may be applied in any suitable order to produce eitherpositive or negative photoreceptors.

The photoreceptors produced by the present disclosure can be utilized inan electrophotographic imaging process by, for example, first uniformlyelectrostatically charging the photoreceptor, then exposing the chargedphotoreceptor to a pattern of activating electromagnetic radiation suchas light, which selectively dissipates the charge in the illuminatedareas of the photoreceptor while leaving behind an electrostatic imagein the non-illuminated areas. This electrostatic latent image may thenbe developed at one or more developing stations to form a visible imageby depositing finely divided electroscopic toner particles, forexamples, from a developer composition, on the surface of thephotoreceptor. The resulting visible toner image can be transferred to asuitable receiving member, such as paper. The photoreceptor is thentypically cleaned at a cleaning station prior to being recharged forformation of subsequent images.

While particular embodiments have been described, alternatives,modifications, variations, improvements, and substantial equivalentsthat are or may be presently unforeseen may arise to applicants orothers skilled in the art. Accordingly, the appended claims as filed andas they may be amended are intended to embrace all such alternatives,modifications variations, improvements, and substantial equivalents.

1. A process for forming a barrier layer on a photoreceptor substrate byorganic acid anodization, the process comprising: providing aphotoreceptor substrate; providing an organic acid electrolyte; applyinga multi-step voltage profile to the photoreceptor substrate in contactwith the electrolyte, wherein the profile includes a first step in whicha first voltage is applied for a first time period and a second step inwhich a second voltage, less than the first voltage is applied for asecond time period.
 2. The process of claim 1 wherein the second voltageis from about 50% to about 20% of the first voltage.
 3. The process ofclaim 1 wherein the second voltage is from about 40% to about 30% of thefirst voltage.
 4. The process of claim 1 wherein the sum of the firsttime period and the second time period is less than 5 minutes.
 5. Theprocess of claim 1 wherein the sum of the first time period and thesecond time period is less than 1 minute.
 6. The process of claim 1wherein the profile includes a third step in which a third voltage, lessthan the second voltage, is applied for a third time period.
 7. Theprocess of claim 6 wherein the third voltage is from about 30% to about20% of the second voltage.
 8. The process of claim 6 wherein the secondvoltage is from about 40% to about 30% of the first voltage.
 9. Theprocess of claim 6 wherein the sum of the first time period, the secondtime period, and the third time period is less than 5 minutes.
 10. Theprocess of claim 6 wherein the sum of the first time period, the secondtime period, and the third time period is less than 1 minute.
 11. Theprocess of claim 1 wherein the first step employs a voltage from about20 to about 24 volts and the second step employs a voltage from about 12to about 17 volts.
 12. The process of claim 6 wherein the first stepemploys a voltage from about 22 to about 26 volts, the second stepemploys a voltage from about 14 to about 18 volts, and the third stepemploys a voltage from about 10 to about 14 volts.
 13. The process ofclaim 1 further comprising: prior to applying the multi-step voltageprofile, subjecting the photoreceptor to a heating operation in whichthe photoreceptor is subjected to a temperature of from about 450° C. toabout 650° C.
 14. The process of claim 13 wherein the photoreceptor issubjected to a temperature of about 550° C.
 15. The substrate andbarrier layer produced by the method of claim
 1. 16. A process forforming a barrier layer on a photoreceptor substrate by organic acidanodization, the process comprising: providing a photoreceptorsubstrate; providing an organic acid electrolyte; heating thephotoreceptor substrate to a temperature of from about 450° C. to about650° C.; contacting the photoreceptor substrate with the electrolyte;applying a voltage to the photoreceptor substrate in contact with theelectrolyte for a period of time so as to form an anodized layerthereon.
 17. The process of claim 16 wherein the temperature is about550° C.
 18. The process of claim 16 wherein the heating is performed fora time period of from about 10 seconds to about 60 seconds.
 19. Theprocess of claim 16 wherein the applying a voltage is performed byapplying a multi-step voltage profile to the photoreceptor substrate,wherein the profile includes a first step in which a first voltage isapplied for a first time period and a second step in which a secondvoltage, less than the first voltage is applied for a second timeperiod.
 20. The substrate and barrier layer produced by the method ofclaim 16.